1. Field of the Invention
The present invention relates to an apparatus for automatically controlling the starting and the subsequent dimming of a plurality of gas discharge lamps using standard commercially available lamp/ballast combinations. More specifically, but not by way of limitation, the apparatus allows gas discharge lamps (fluorescent, mercury vapor, metal halide, or high pressure sodium) to be started at full power and then automatically dimmed with no further operator intervention. Furthermore, after power is restored following a momentary power outage, the apparatus will restart the lamps at full power and then automatically readjust them to their prior dimmed setting.
2. Description of the Related Art
Various methods and apparatus presently exist to start gas discharge lamps. One such method starts fluorescent lamps utilizing starter switch "S" connected to both electrodes of a lamp (see FIG. 1). Initially, starter switch "S" is closed in order to conduct the applied electrical current to each electrode. Current is applied directly to each electrode so that the electrodes will be heated to thermionic emission temperature. Once the electrodes reach their thermionic emission temperature, the switch is opened, and the lamp starts.
However, because switches utilized in the above method and apparatus must be periodically replaced, starterless circuits have been devised. A starterless circuit such as that shown in FIG. 2, applies AC current to a lamp ballast which may be either a conventional transformer or an autotransformer. In either case, a portion of the transformer's output is tapped for each electrode to produce a current that heats the electrodes to thermionic emission temperature. Again, once the electrodes reach thermionic emission temperature, the lamps will start.
Gas discharge lamps such as high pressure sodium (HPS) lamps also require a starter switch to provide an initial high voltage pulse across both electrodes of the lamps. In the starter switch shown in FIG. 4, the output of the ballast is applied to an igniter which applies full power to both electrodes of the lamp, thereby heating the electrodes to thermionic emission temperature and starting the lamp.
For reasons of both energy efficiency and consumer preference, it is desirable to incorporate dimming devices into fluorescent and high intensity discharge (HID) lamp circuits. Such dimming devices, which are well known in the art, generally use solid-state components such as silicon controlled rectifiers (SCRs) or triacs to block portions of each half-cycle of the incoming AC voltage. By only allowing portions of each half-cycle to be conducted to the ballast, the amount of power delivered is thereby reduced.
However, a problem with such dimming devices, when applied in gas discharge lamps, is that full power is needed to start each lamp because of their high thermionic emission temperatures, and the particular starters required as discussed above. Additionally, after any power interruption, even a momentary one, all HID lamps (mercury vapor, metal halide, and high pressure sodium) require a cool down period during which full power is supplied before they will restrike. That period varies from about one minute for high pressure sodium lamps to between ten and fifteen minutes for metal halide lamps. Thus, the above dimming devices when activated will not be able to originally start or restart HID lamps. To start the HID lamps, a user must first manually apply full power to the ballast to start the lamps and then manually adjust the dimming device to the desired level. The dimming device, therefore, cannot be left at the desired setting because at every start-up or restart, a manual readjustment is required.
In an attempt to solve the above problem, an apparatus that utilizes special fluorescent ballasts (see FIG. 3) has been developed. The special fluorescent ballasts are provided with sections which continuously furnish power to the cathodes of the lamps, thereby maintaining the thermionic emission temperature of the cathodes. Unfortunately, these devices are costly and require extra wiring to control the dimming of the lamp.
Additionally, other devices currently exist for regulating gas discharge lamps. Such devices are typically installed in series between the ballast and the lamp. These devices range from the addition of one or more capacitors or reactive devices to the installation of more sophisticated solid-state circuitry such as that disclosed in U.S. Pat. No. 4,147,961, issued on Apr. 3, 1979 to Elms, and U.S. Pat. No. 4,147,962, issued on Apr. 3, 1979 to Engel.
Unfortunately, the devices disclosed in the above patents suffer several disadvantages. First, installation requires opening each fixture and installing additional components, thus significantly increasing initial costs. Second, extra control leads may be required in each fixture to allow a variance in the energy savings or lighting levels. That requirement makes a retrofit application very costly because it is often difficult to pull additional conductors through a conduit already filled to capacity. Furthermore, both the lamp fixtures and the conduits could be at a height requiring special equipment for installation. Third, the number and cost of individual components susceptible to failure create maintenance problems. Finally, and most importantly, full power will most likely not be available to the lamps at start up, resulting in a failure of the devices to start the lamps.
A dimming device which attempts to deal with the problem of starting of gas discharge lamps is disclosed in U.S. Pat. No. 4,287,455. The '455 device employs power switch SCRs that are gated by a unijunction transistor via a control switch SCR and a pair of optocouplers. To produce appropriate phase control, the power switch SCRs are non-conducting until the unijunction transistor fires a pulse. The unijunction transistor, however, will not fire until a group of capacitors connected to its emitter are charged to a required value. The charging of the capacitors is accomplished by transforming and rectifying the incoming AC voltage using a transformer and rectifying bridge circuit, and then feeding the rectified voltage to the capacitors through a potentiometer. Thus, the phase portion of the incoming AC voltage which is conducted to the load depends upon the values of the capacitors and the setting of the potentiometer. To provide automatic starting when the device is set to dim the lamps, the capacitors coupled to the unijunction transistor also receive charging current through a charging diode that is further connected to the collector of a starting transistor. Additionally, a pair of capacitors is connected to the base of the starting transistor. When the pair of capacitors is charged, the transistor is turned on which reverse biases the charging diode, thus, no longer enabling the capacitors connected to the unijunction transistor to be charged via that pathway. Therefore, when power is initially applied to the device, the capacitors coupled to the unijunction transistor are charged via both the charging diode pathway as well as the potentiometer pathway. That supplemental charging causes the triggering of the unijunction transistor earlier in the AC cycle than in normal dimming operation. The '455 device is supposed to deliver enough power to start the lamps before the starting transistor turns on.
However, full power is never directly applied to the load during the starting operation. Instead, the power switch SCRs are simply triggered earlier in the AC half-cycle than in the normal dimming operation. Thus, whether enough power is delivered to start the lamps depends upon the load. That is, with enough lamps connected to the device, it will be unable to supply sufficient power to light all the lamps. In that situation, only full power will start the lamps.
Furthermore, a portion of the charging current for the capacitors connected to the unijunction transistor always flows through the potentiometer. At a high dim setting, the triggering of the unijunction transistor occurs late in the AC cycle, even during the starting operation. Again, depending on the load, the resulting amount of power transmitted may not be enough to start the lamps. Thus, the '455 device makes it impractical to furnish full power to HID lamps for the time necessary to allow the cool-down period before the lamps will restrike. Additionally, the capacitors connected to the starting transistor must discharge enough to turn off the starting transistor before the starting operation can take place. That is supposed to occur whenever power is removed from the device, however, a power outage of sufficiently short duration may extinguish the lamps while preventing the capacitors from sufficiently discharging. In that case, the lamps would have been restarted manually.
Another dimming device is disclosed in U.S. Pat. No. 5,043,635, issued on Aug. 27, 1991 to Talbott. It has similar shortcomings as the device described above. That is, if a power outage of only a few seconds occurs, the lamps under the control of '635 device must be restarted manually.
Another dimming device is disclosed in U.S. Pat. No. 4,950,96 issued to Sievers, the inventor of the present invention. During sustained operation, the '963 device provides lamp dimming by delivering power to the lamp only during portions of the AC cycle. Timing capacitors contained in a bridge circuit, when charged to the threshold voltage of a diac, discharge during a half-cycle into the gate of a triac causing the triac to conduct the incoming AC line to the lamp. Diodes, also contained in the bridge circuit, allow the timing capacitors to be reset during the opposite half-cycle. The '963 device is further provided with potentiometers that are used to adjust the charging times of the charging capacitors and, therefore, the amount by which the lamp is dimmed. However, during system power up or a momentary power failure, the '963 device provides full power to light or relight the lamp. Upon power up, a timer is initialized and begins operation. While the timer is operating, a signal turns on a transistor which turns on an opto-isolator causing the triac to deliver the full wave AC power to the lamp. After the timer times out, the transistor is turned off, thereby allowing the bridge circuit to automatically take over to deliver the reduced AC power.
The '963 device is an improvement over conventional dimming devices because the amount the lamp is dimmed after the full power warm up period expires may be set once and will remain set without further adjustment. Unfortunately, because the '963 device goes from a full power warm up period directly to its dimmed level in a one-step reduction of power, it does not operate properly when used to control HID lamps. When the '963 device is used with high pressure sodium lamps, many lamps may extinguish after the one-step dimming process.
All of the above described dimming devices, by utilizing SCRs or triacs, introduce harmonics into the system and result in low power factor operation of the circuit. Both factors cause problems with the utility company serving the particular installation using a dimming device. In some instances, the use of the dimming devices affects the system to such an extent that additional charges may be accrued which offset any savings realized.